Restore peak compression and dependable power to Yamaha YFM/YFB 250 ATVs with the OS 1.00 Piston Kit. This oversize bore kit is engineered for Timberwolf, Bear Tracker, and Bruin models, covering 1989–2001 and 2005–2006, delivering a +1.00 mm bore to compensate wear and reclaim factory-like response. The kit arrives as a complete top-end refresh package, designed for a straightforward install so you can hit the trail or farm hard without lengthy downtime.

Over time, piston-ring wear, bore taper, and ring-land scuffing sap compression and throttle response, especially under heavy trail work or plowing. The OS 1.00 piston kit targets these wear points with a forged aluminum piston and a full ring-set, wrist pin, and circlips to restore compression, heat tolerance, and consistent power. Built as an all-in-one aftermarket replacement, it delivers reliable, repeatable performance in demanding Yamaha YFM/YFB 250 applications.

Comprehensive Vehicle Fitment

Product Specifications

OEM & Aftermarket Cross-Reference Information

What's Included

All essential components for a complete top-end refresh are included.

• Piston
• Rings
• Wrist Pin
• Circlips

Related gaskets, seals, and top-end hardware are not included and are typically purchased separately.

Why Choose This Aftermarket Replacement?

• Direct-Fit Compatibility for Yamaha YFM/YFB 250 engines ensures straightforward installation with proper bore and ring alignment.
• OEM Cross-Reference provides clear alignment with Yamaha’s original part numbers while delivering a ready aftermarket alternative.
• Premium Materials—forged aluminum piston delivers strength, durability, and heat resistance in demanding riding conditions.
• Oversized Bore (+1.00 mm) to restore compression and power after wear, extending engine life between rebuilds.
• All-in-One Top-End Refresh—includes piston, rings, wrist pin, and circlips for a complete, field-ready package.
• Reliability in Tough Terrain—engineered to withstand trail riding, work duty, and farm chores with consistent performance.

Installation Notes:

• Inspect the cylinder bore for scoring and taper; verify bore size is within oversize kit specifications before installation.
• Use a proper ring compressor to install the piston; ensure rings are oriented correctly and gaps are staggered per OEM guidelines.
• Lubricate the piston, rings, and wrist pin with assembly lube before final assembly to prevent dry-start wear.
• Install the wrist pin and circlips carefully; confirm clips are fully seated and not projecting into the crankcase.
• Clean all gasket surfaces and ensure the cylinder head is clear of debris; replace gaskets if reused during top-end service.
• Follow a cautious break-in procedure: throttle lightly for the first 20–30 minutes, gradually increasing RPM to seat rings and verify compression before sustained high-rpm operation.

Why Upgrade?

Running a worn piston kit drains compression and throttle response, leading to reduced power, increased fuel consumption, and higher risk of cylinder damage. This OS 1.00 oversize kit restores proper ring seal and bore geometry, delivering reliable compression and predictable performance for trail riding and farm work. Upgrading now helps prevent sudden power loss and preserves engine longevity.